This invention relates to apparatus for use in a liquid alkali metal environment.
The use of liquid alkali metal such as sodium in industry is now quite common. For example, in the nuclear reactor industry liquid sodium is used as a vehicle to convey heat energy from a nuclear reactor core to heat exchange apparatus wherein steam is generated and superheated. Because of the hostile nature of sodium, particularly when operating in the temperature range of 500.degree.-600.degree. C., wear, fretting and high coefficients of friction between bearing surfaces present a serious problem. For example, where alloy steel components make bearing contact with complementary bearing surfaces there is a tendency for transfer of the alloy steel to the complementary bearing surface. Several materials have been used or suggested for the complementary bearing surfaces, for example, stabilised 21/4Cr 1 Mo stainless steel, nickel base alloys such as Inconel 718 (Inconel is a registered Trade Mark), stellite and chromium carbide but none of these bearing surface materials is totally satisfactory when used with alloy steel.
U.S. patent Ser. No. 4,294,659 discloses apparatus wherein one bearing surface is of alloy steel and the complementary bearing surface of a nickel base alloy component is aluminised. With such apparatus wear of the surfaces is considerably reduced when compared with that in previously known apparatus; the wear tracks remain relatively smooth and the coefficient of friction is relatively low. It is believed that the exceptionally good behaviour of these materials is brought about by the relatively easy formation of sodium aluminate by reaction with the oxygen containing sodium; the sodium aluminate serves as a lubricant. However, the process of aluminising the bearing surfaces, that is, the diffusing of aluminium into the surfaces, is a high temperature one and such high temperatures can be harmful to the treated component.